BOD or Biochemical Oxygen Demand (Also known as Biological Oxygen Demand), is the amount of oxygen needed by aerobic organisms to break down the organic substances in sewage water. Polluted water will have higher BOD. For instance, if BOD of water is 1-2, the water quality is very good. BOD of poor quality of water is 6-9 and BOD of highly polluted water would be more than 17. When BOD levels are high, dissolved oxygen (DO) levels decrease because the oxygen that is available in the water is being consumed by the bacteria due to which aquatic organisms will not get oxygen for survival.